Why do apparently healthy looking citrus trees suddenly start deteriorating in a matter of days, with leaves curling then dropping, branches dying back, eventually resulting in the loss of the tree? There are several causes, often caused by common gardening mistakes, which are easy to avoid.
Curling Leaves as an Indicator of Root Problems
If the leaves of a citrus tree are curling evenly or cupping along their length, the tree is trying to minimise the surface area of the leaf in order to reduce transpiration and moisture loss, which is an indicator that the roots can’t drive enough moisture to the leaves. Root function is impaired when the roots have been damaged in some way, and this is often due to lack of water or overwatering, both of which cause some of the roots to die back to the extent that they can no longer support the full canopy of leaves.
How do we tell if the tree has run dry or is waterlogged? Simply push a garden trowel (small hand spade) the full length of the blade into the soil, and pull it back to observe the soil underneath. Is the soil bone-dry, like powder or waterlogged? Some soils can dry to the point that they become water repellent deeper down, even though the surface above is damp, while others may be even wetter below, and really muddy.
If leaves are curled on one side, or completely rolled up quite tightly, it’s usually an indication of a pest such as an aphid or caterpillar, and if the leaf is deformed and oddly curled, with silver tracks inside the leaf, then the problem is leaf miner. Treat these pests with a horticultural oil, use the vegetable-based varieties if you’re an organic gardener as the synthetic ones are just paraffin oil which is persistent and clogs up the soil.
When young leaves are attacked by citrus leaf miner they become twisted and deformed.
Sometimes, citrus leaves will curl when there’s extreme weather conditions, such as heat, cold and wind, so check the soil to be sure!
Citrus in Pots and Containers
If a citrus tree is not in the ground but in a pot, then there should be no problem with root rot, as pots have drainage holes, unless… the pot is sitting in a tray of water!!!
When citrus in a pot or container sits in a tray of water, the bottom level of potting mix will wick up water like a sponge to create a perched water table and become waterlogged. Without oxygen in the root zone, the roots will rot, and if root rot sets in the tree will die. For this reason, never plant a citrus tree in a self-watering pot, they have a water reservoir tray underneath which holds water and wicks upwards, causing the potting medium to stay overly wet, especially in winter when trees are dormant and don’t need much water.
DO NOT USE SELF-WATERING POTS FOR CITRUS OR ANY OTHER FRUIT TREE!
Some edible plants such as mint, taro, water chestnuts, watercress, pennywort, Vietnamese mint and brahmi for example, can sit in trays of water, and enjoy it. Other plants such as sugarcane can also sit in water for short periods of time because they’re very thirsty plants which take up the water very quickly and use it up. Fruit trees aren’t like any of these plants, and neither are citrus. I hope I’ve emphasised the point enough.
Sugar cane (Saccharum officinarum) plant growing in a pot sitting in a tray which retains water, because sugar cane is a one of the thirstiest crops on the planet, do not do the same with a citrus tree in a pot or the roots will rot!
It’s customary to place a water tray under a potted fruit tree if it’s sitting on a deck or balcony to prevent water stains from watering. In such instances, use a paver or terracotta pot feet under the pot to elevate it above the water level. This way the tray catches the excess water when watering but the pot doesn’t sit in the water.
Avoiding Root Rot
When planted in the ground, citrus trees should be planted in a location that drains well. Avoid locations where water tends to gather or stagnate, and in heavy clay soils, improve drainage by mixing compost into the planting hole.
Avoiding Root Burn
Roots will ‘burn’ when they run out of water, and citrus is quite shallow rooted, so it’s fairly easy for citrus trees to dry out in hot, windy, exposed locations and in sandy soils. In the case of sandy soils, improve water retention by mixing compost into the planting hole, compost fixes everything! To reduce water evaporation from the soil, mulch around and under citrus trees in late spring once the soil has warmed up. Keep mulch material a few centimetres away from the trunk to prevent collar rot which can ringbark the tree and kill it.
Another thing that can take moisture out of tree roots and cause them to burn is too much fertilizer. Overfertilizing is as bad as underfertilizing, perhaps worse. Chicken manure has high levels of mineral salts (not mineral salt, there’s a difference!) so when it’s piled on thick or overapplied, it draws water out of the roots by osmosis, causing root burn. The same thing happens if any synthetic mineral fertilizers are use in excess. Always follow the recommended fertilizer application rates, they’re stated for a reason.
Killer Chickens and Dangerous Gardeners?
Citrus tree roots don’t run very deep, only about 50cm (20”), and they have a lot of feeder roots just below the surface. Chickens love to mess up the surface of the garden, looking for things to eat, they scratch and dig very effectively, enough to severely disrupt the delicate surface roots. When this happens, the tree may show drooping leaves the next day. Yes, your chickens can kill your lemon tree, or any other citrus tree for that matter. Citrus trees roots can be protected from chickens scratching up the soil by placing plastic or galvanised wire mesh flat on the soil surface around the root area, and covering it with mulch so it’s not visible. The mesh can be effectively fastened in place by using weedmat or irrigation pins,
Overzealous gardeners can be equally dangerous to citrus trees. Only plant very shallow rooted plants under citrus, such as thyme for example, don’t dig around the root zone if possible, one plant added every now and then and the tree has time to recover from the root damage, a mass planting on the same day will cause too much root damage and kill the tree.
Be careful when people ask if they can ‘help’ in the garden without supervision. Ten year old Lisbon lemon tree dead, no surface roots evident – this is what happens when the area under the tree is cleared and planted up all at once with small annual flower seedlings… thanks mum!!!
So go ahead and plant your favourite lemon, lime, orange, mandarin, grapefruit, cumquat, or whatever takes your fancy, whether it’s in a pot or in the ground. With proper care and regular feeding (at the start of spring and autumn with a balanced fertilizer), citrus trees can be productive for decades